1. Field of the Invention
The present invention generally relates to a display device and more particularly, to a liquid crystal display panel employing active elements.
2. Description of the Prior Art
Conventionally, as a transistor array for a liquid crystal display device used for the above purpose, there has been provided an arrangement as shown in FIGS. 2(a) and 2(b), and disclosed, for example, in a paper SID 89 DIGEST p.114 "Rear-Projection TV using High-Resolution a-Si TFT-LCD". More specifically, the array substrate as referred to above includes a gate electrode 4 for scanning lines X1-XM, a source electrode 6 for signal lines Y1-YN, and further, thin film transistors (referred to as TFTs hereinafter) 2 formed corresponding to respective intersections, with a drain electrode 7 of the respective TFT being connected to a pixel electrode 3. A liquid crystal material 13 is inserted between a substrate 1 constituting the TFT and a confronting electrode 14, and independent pixels are formed between the pixel electrode 3 and confronting ground electrode 16 provided on the confronting substrate 14, with the liquid crystal material 13 functioning as a capacitance equivalently. There are many cases where an auxiliary capacitance 12 is added in an electrically parallel relationship thereto depending on the need for improving the holding of the signal voltage. For constituting such an auxiliary capacitance, it has been recent trend to form the auxiliary capacitance 12 between the scanning lines 5Xi-1 at a previous stage and said display pixel electrode 3 in order to reduce the number of masks.
However, in the case where the liquid crystal display mode is driven by a normally white mode (referred to as an N.W. mode hereinafter) in which the display is made black during the application of a voltage and is made white during the non-application of a voltage, due to the occurrence of non-uniformity in the electric field within the pixels, there has been a problem in that consequent non-uniformity takes place in the orientation of liquid crystal molecules within the image area, thus resulting in a lowering of the display quality arising from visible irregularity of the display.
Still referring to FIGS. 2(a) and 2(b) showing the conventional TFT-LCD of the previous stage capacitance type, problems taking place in the planar and sectional constructions in the known arrangement will be described hereinafter.
In the conventional arrangement formed with the previous stage auxiliary capacitance 12, the display pixel electrode 3C (i, j) does not perfectly cover the scanning electrode 5Xi-1 at the previous stage in the direction of the line width, and thus, an edge portion of the previous stage electrode 5Xi-1 located close to said display pixel electrode 3C (i, j) is exposed. The exposure of the scanning electrode 5Xi-1 in terms of plane through the insulative layer results in the local lowering of transmission factor due to the non-uniformity of signal potential within the pixels arising from the leakage of the signal potential of the gate into the pixels. More specifically, in the above known arrangement, on the assumption, for example, that the signal applied to the pixel 3C (i, j) is at 4 V, and the potential of gate electrode 4Xi and 5Xi is at -9 V as an off potential, and the potential of a confronting substrate 14 is at 0 V, the the potential distribution in the liquid crystal 13 constituting the pixels 3C (i, j) will be represented as shown in FIG. 3, and thus, the non-uniformity of electric field distribution within the pixels 3C (i, j) becomes larger, thereby forming a distribution within one pixel face in a transmittance characteristic of light.
Accordingly, it has been necessary to hide the display from the pixel edge as much as 8 .mu.m in total, i.e., about 4 .mu.m from the edge of the pixel electrode 3 as the non-uniformity portion of the signal potential, and also, about 4 .mu.m as a margin for combining a black matrix disposed at the confronting substrate 14 and the pixel electrode 3. Therefore, there has been a problem in that display aperture ratio of the pixel is lowered as the density becomes higher.